In general, cylinder body and plug of known lock cylinder structure setup pins including a plurality of upper pins of same height disposed within the cylinder body and a plurality of lower pins of different height disposed within the plug. Combination of the upper and lower pins is utilized to limit the plug not to rotate within the cylinder body so as to perform functions of limitation and rotation-proof. It needs to insert a proper key into the plug as opening lock, in which the proper key has a bitting with different height for engaging with the lower pins with different height. When the proper key is inserted into the plug, top surfaces of every lower pins of the plug are roughly aligned with the interface between the cylinder body and the plug allowing the plug not to be limited by the upper and lower pins capable of rotating within the cylinder body. However, when a burglar attempts to illegally open a lock, he may insert a tool into keyhole of the plug to force pins bouncing upward and rotate the plug timely thereby easily opening lock, so burglarproof and security are inefficient needed to be improved.